text: Using the number density as a function of spatial coordinates, the total number of objects N in the entire volume V can be calculated as
  where dV = dx dy dz is a volume element. If each object possesses the same mass m, the total mass m of all the objects in the volume V can be expressed as
  Similar expressions are valid for electric charge or any other extensive quantity associated with countable objects. For example, replacing m with q (total charge) and m with q (charge of each object) in the above equation will lead to a correct expression for charge.
  The number density of solute molecules in a solvent is sometimes called concentration, although usually concentration is expressed as a number of moles per unit volume (and thus called molar concentration).